Metering pumps of this type have been known. They are used for both the metered dispensing of a single medium and the simultaneous metered dispensing of two different media, the medium to be dispensed being accommodated in a container that is or can be connected to the metering pump. There are dispensing devices, so-called dispensers, with two separate storage containers and two metering pumps, which can be actuated together by means of an actuating lever or the like (DE 101 10 888 A1), as well as dispensers in which two different media are accommodated in a single storage container (DE 200 19 540 U1) and are delivered by two separate metering pumps in different metered quantities to a common discharge opening.
The bellows as the pumping member plays functionally the most important role in metering pumps of this type. This bellows also must consist of plastic as an injection-molded part and be able to apply the restoring force for the resetting into the starting position, which is necessary after each working stroke, in order to draw from the storage container the quantity of medium that had been dispensed during the working stroke into the pump chamber in the process. The bellows must therefore have a certain minimum size in terms of both its axial length and the number of elastic bellows folds.
However, this also results in a corresponding minimum size of the interior space of the bellows that forms the annular chamber, which corresponds in some applications to a multiple of the quantity to be dispensed during each stroke of the pump. To reduce the volume of the pump chamber, which is determined by the minimum size of the bellows, a displacement piston, which protrudes into the upper part of the pump chamber or the bellows, is already provided in these prior-art metering pumps.
Due to the reduction achieved in the volume of the pump chamber, an improvement of the suction process is also achieved during the first pump strokes, during which the bellows is still filled first with air.
However, the displacement piston in these prior-art metering pumps has a central opening at its lower front side, through which the medium to be pumped is pumped to the dispensing opening to the discharge valve or through the said discharge valve. A residual air cushion is therefore inevitably formed around the displacement piston in the upper area of the bellows in these prior-art metering pumps. The consequence of this compressible air cushion may be that the quantities dispensed during the individual pump strokes will be different.
The phenomenon of the different dispensed quantities is not noticed by the user in case of an individual metering pump, which delivers only a single medium.
However, different dispensed quantities are perceptible in case of dispensers with two metering pumps and storage containers arranged next to each other, from which medium is being delivered separately, with metering pumps that have completely identical design and also work in the same manner if the axial positions of the two follower pistons are different already after a number of pump strokes in case of transparent or translucent container walls. In addition, this drawback can also be recognized from the fact that one storage container is pumped empty sooner than the other one.